Olivier Cochard-Labbe, known as the FreeNAS founder and a FreeBSD contributor, has released a paper about routing and firewalling for FreeBSD operating system. In this document, Olivier goes into detail through the basic concepts, tuning forwarding performance, and firewalls performance. See the link below for the full document.

Tuning FreeBSD for routing and firewalling by Olivier: https://people.freebsd.org/~olivier/talks/2018_AsiaBSDCon_Tuning_FreeBSD_for_routing_and_firewalling-Paper.pdf

Related links:
https://medium.com/@matteocroce/linux-and-freebsd-networking-cbadcdb15ddd?fbclid=IwAR1Z5WOODJWTey3eEQZEtHGUP7uQTczcwLAT4qyLOa2Dlm_yBj-80mTwahM

Source: https://forum.opnsense.org/index.php?topic=10356.0#msg_47699_extra_info